Continuing requirements to achieve improved fuel efficiency and reduced exhaust gas emissions of compression ignition engines, hereinafter referred to as diesel engines, has stimulated the development of electronically controlled fueling systems offering the potential for providing more precise engine control. The gains achievable in diesel engine performance through the introduction of electronic fuel controls depend to a great extent on the control strategy implemented, the accuracy to which specific engine operating parameters can be measured and controlled and the ability to maintain such control throughout the operational life of the engine.
In compression ignition engines, one of the most critical operating parameters is fuel injection timing. Presently, control of the time of injection is determined mechanically and/or hydraulically. The timing function has typically relied only upon measurements of mechanical timing points, such as crank angle, flywheel position, piston position and/or injector actuation to provide the requisite timing control. While such control was historically effected mechanically and/or electromechanically, recent developments have placed increasing emphasis on the utilization of electronics. Representative of these timing techniques and implementations are U.S. Pat. Nos. 4,033,310 and 4,265,200 which sense injector actuation to provide corrective feedback information to electronic controls which determine and control the timing of fuel delivery, or injection, by fuel delivery apparatus.
Those systems, however, fail to provide for the fact that in diesel engines, unlike spark ignition engines, the start of combustion within the cylinder does not directly relate under most circumstances to the mechanical timing point, such as injector actuation. Engine operating conditions such as cylinder wall temperature, air inlet temperature, engine load and speed and fuel quality all influence the specific point or time in the engine cycle at which combustion takes place within the cylinder. An additional complication is the contemplated introduction of a broad spectrum of new fuels, fuel blends (i.e. alcohol and water emulsions), and synthetic fuels widely ranging in cetane rating. These factors combine to introduce a variable delay between the time of fuel injection and the start of combustion which may typically be 5.degree.-20.degree. of crank angle. To accommodate such variations in the onset of combustion introduced by the above factors, the purely mechanical timing system must be augmented with precise information on the aforementioned engine operating parameters, as well as with a direct measurement of fuel quality (cetane rating) and fuel density. From this information, it then becomes possible to estimate the instant at which combustion begins. Obviously, the complexity of this approach along with the large number of required sensor inputs limits accuracy and practicability. Furthermore, this approach can, at best, provide only an estimate of the onset of combustion and cannot provide compensation for engine variables.
While the introduction of electronic control systems to diesel engines is relatively new, considerable development has occurred with spark ignition gasoline engines. Specifically, efforts have been made to improve spark ignition engine performance via the electronic controls associated with engines. For instance, in U.S. Pat. No. 4,181,944, which in turn refers to a different Japanese patent application KoKai (laid-open) No. 4903/72, there is a general discussion of using combustion pressure sensors for monitoring the pressure in one or more engine cylinders and for modifying a previously-stored spark ignition timing scheme if the sensed pressure indicates deterioration of the cylinder pressure. Mention is also made of sensing the ion current in the spark plugs in lieu of a pressure measurement. These techniques, however, are intended for use with spark ignition engines and do not sense the timing of the combustion event, but rather its quality.
Various techniques other than an analysis of pressure have also existed for indicating some combustion-related characteristics of an engine. Two such examples, U.S. Pat. Nos. 2,523,017 and 4,232,545, utilize an ionic current detector to detect knocking or "detonation" in a spark ignited engine, either for analytical or corrective control purposes. U.S. Pat. No. 3,051,035 describes an optical combustion monitoring device for detecting a flame-out condition in aircraft jet engines. However, these patents are not concerned with the timing of the onset of combustion nor with the development of a timing signal for a diesel engine, nor specifically with control of fuel injection timing based on a direct measurement of the onset of combustion.
Accordingly, it is a principal object of the present invention to provide improved control of the timing of fuel delivery in diesel engines. Included within this object is the provision of a method and apparatus for controlling such fuel delivery in an accurate and precise manner as a function of the onset of combustion in the engine.
It is a further object of the invention to provide apparatus for accurately sensing the onset of combustion and generating corresponding start-of-combustion timing signals therefrom. Included within this object is the provision of such apparatus which is relatively durable and long lived, yet relatively inexpensive.
In accordance with one aspect of the invention, there is provided the method of and apparatus for controlling fuel delivery in a compression ignition engine at least partly as a function of the onset of combustion in the engine. Command signals indicative of the desired start-of-combustion timing are provided as a function of engine operating parameters and are utilized in open-loop manner to control the timing of fuel delivery. The command signals are modified or trimmed as necessary to correct for the variable delays which generally occur between the time (i.e. engine crank angle) of the fuel delivery and the start of combustion. The appropriate correction of those control signals is achieved by detecting the actual instant of the start-of-combustion in a respective combustion chamber, comparing that actual time (i.e. crank angle) with the time which was desired, thereby to detect any error, and correcting the original control signal by an amount equal to or proportional to the error. The desired start of combustion values may be previously determined and stored for a full range of engine speeds and loads. The correction signals may also be stored as a function of engine speeds and loads and may be periodically updated by the determined error values. The processing of error values is done in a manner providing dynamic and accurate correction for the control signal even though non-monitored engine operating conditions may change. Provision is made for a cold-starting advance.
In accordance with another aspect of the invention, a signal generator is provided for responding to a direct property of the combustion occurring in a cylinder to generate a timing signal indicative of the onset of combustion. A sensor in communication with a cylinder combustion chamber detects the particular property of combustion being monitored, the level of that detected property normally changing at a rapid rate, typically increasing, at the onset of combustion. The sensed property is then converted, as by signal conditioning means, to an electrical timing signal which accurately indicates the onset of combustion.
In one embodiment, the sensor is optical in character and senses electromagnetic radiation, i.e. light of some frequency or frequency range, emitted by the combustion event. A photodiode provides an electrical analog of the sensed light. Signal conditioning circuitry then squares the leading edge of the electrical analog, which leading edge then is indicative of the onset of combustion and is used in controlling the timing of fuel delivery in the diesel engine. The combustion radiation may be sensed by a heat-resistant optical element and coupled, as by a fiber optical cable, to the photodiode.
In another embodiment of the invention, the sensor detects the level of ionization in the combustion chamber. An electrical current is developed and, following the type of signal conditioning described in the preceding paragraph, provides an electrical signal accurately indicative of the onset of combustion. The sensor includes one or more electrodes mounted in a ceramic insulator.
For certain engines, either type of sensor may assume the general form of a glow plug for mounting in the precombustion chamber of the engine. A heating element may be included in the gross sensor structure.
The start of combustion signal generator is employed in combination with the fuel delivery control system of the engine throughout operation of the system to provide dynamic control.